AC/DC converters serve as power supplies for a wide variety of applications. The applications range from power adapters for portable electronic devices to power supplies for networking equipment and home appliances. Typical IC packages used to implement AC/DC converters include a semiconductor power device and a digital controller for controlling the semiconductor power device. Legacy type IC packaging (such as e.g. Small Outline Integrated Circuit SOIC, Plastic Dual In-Line Package PDIP, Thin Shrink Small Outline Package TSSOP, Mini Small Outline Package MSOP, Shrink Small Outline Package SSOP, or dual flat no-leads DFN) is often used.
During packaging, dies i.e. blocks of semiconducting material are encapsulated in the IC package which serves as a supporting case for preventing physical damage and corrosion. Moreover, the IC package supports the electrical contacts (denoted as leads or pins) which connect the dies with a printed circuit board PCB to which the IC package may be attached.
In general, several design considerations need to be taken into account. For instance, the electrical characteristics of current-carrying traces that run out of a die, through the IC package, and into the PCB have to be designed carefully. Specifically, it is desirable that the latter current-carrying traces exhibit low resistance, low capacitance and low inductance.
On the other hand, mechanical reliability is an important design criteria. The IC package is responsible for keeping the chip safe from potential damage. The package must resist physical breakage and provide an airtight seal to keep out moisture. The expansion of trapped moisture can result in internal separation (delamination) of the different materials and structures. Cracks may extend to the surface of the IC package and, in the most severe cases, the entire IC package may bulge and pop. This is known as “popcorn” effect.
Finally, thermal considerations need to be taken into account. It is the duty of the IC package to provide effective heat dissipation away from a die. This holds in particular for the case of semiconductor power devices used in AC/DC converters.